Many websites require user credentials. At the same time, users may have different credentials for different websites and other purposes.
Presenting credentials stored on a computer to a website can cause issues with users. For example, if the credentials are selected and presented to a website entirely outside the browser with which the user is accessing the website, it is often difficult for the user to associate the submission tool with the site. If the credentials are selected and presented within the browser, the user may feel the site already possesses and has access to all of the user's credentials.